This invention relates to bridging plugs for plugging holes in underground formations. More particularly, the invention relates to a bridging plug for closing off a bore hole in preparation for placing an explosive charge in the hole. The present invention is particularly applicable to, but not necessarily limited to, the art of in situ retorting of oil shale.
In mining operations, such as in situ retorting of oil shale, it is often desirable to place explosive charges at desired locations in the oil shale deposit. For example, an in situ oil shale retort is commonly formed by initially removing oil shale from certain portions of the deposit to form vertically spaced apart void volumes or rooms. Explosive charges are then dispersed throughout the portions of the deposit between the rooms, after which these portions are explosively expanded to fill the retort with fragmented oil shale particles.
Explosive charges are placed in relatively small-diameter explosive placement holes, commonly called "blast holes", which extend a relatively long distance through the oil shale deposit. Blast holes are commonly in the range of between about 33/4 inches to 107/8 inches in diameter; and they extend some times as much as several hundred feet through the formation. Inasmuch as an oil shale formation is a relatively impermeable rock mass, these relatively long blast holes are of relatively uniform diameter from top to bottom.
There are various situations in which it is desirable to plug the lower portion of a blast hole and then place an explosive charge in the plugged hole. For example, blast holes are often drilled downwardly from the floor of an upper room through the ceiling of a lower room below the upper room. The blast holes are then plugged to hold an explosive charge in the bottom of the blast hole, especially if the explosive agent is a free flowing explosive such as ammonium nitrate and fuel oil (ANFO).
As a further example, it is often desirable to place explosive charges at selected elevations throughout a formation in order to control the explosive expansion of the deposit. Placement of explosive charges at selected elevations can be accomplished efficiently by drilling the blast holes and then plugging the holes at the desired elevations prior to placing the explosive charges in the plugged holes.
Blast holes also are plugged at locations selected to ensure that water soluble explosives, such as ANFO, do not come into appreciable contact with water-bearing portions of a deposit.
In the past, there have been disclosed a number of bridging plugs for plugging the bottoms of holes drilled in underground formations. The following patents illustrate such bridging plugs:
______________________________________ U.S. Pat. No. Patentee ______________________________________ 1,092,540 Robinson 2,047,774 Greene 2,253,224 Bleakley 2,710,065 Hamilton, Jr. 3,039,534 Koop 3,126,827 McReynolds, Jr. 3,170,516 Holland et al 3,593,785 Bassani 3,674,088 Ovelson ______________________________________
The bridging plugs disclosed in these patents are generally complex in structure and relatively expensive to manufacture when compared with the present invention. As a result, they are not practical for uses involving large scale blasting operations, such as those used in forming an in situ oil shale retort. Moreover, the complex structures of the bridging plugs disclosed in these patents make them relatively difficult, if not impossible, to use in the narrow-diameter blast holes commonly used in in situ retorting of oil shale.
The present invention overcomes these disadvantages by providing a relatively simple, but highly effective, hinged bridging plug which is especially useful in plugging the bottoms of relatively small diameter blast holes.